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Old 12th January 2010, 04:02 AM   #1
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Default Transformer query

I have a basic question about loading a transformer to drop the AC secondary voltage. I have a P Tech P-3A DAC that is powered by a cheap transformer that gives the required 9 VAC (not DC). I've put the meter across the secondary (of the transformer) when the DAC is on and the voltage is 10.28 VAC with the current draw at 0.52 amps - the voltage seems a little high but the DAC works OK with it.
With the aim to upgrade the transformer, I've bought a flat-pack transformer (10 V @ 1.2 amps) but the problem is that with the new transformer connected (to the DAC) the AC voltage is around 11.5 volts - way too high, I think.
To drop the secondary voltage , should I load the transformer with a resistor across the secondary output (with the DAC connected) to bring it down to around 10 VAC? Or any other ideas?

Thanks
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Old 14th January 2010, 07:12 AM   #2
Mooly is offline Mooly  United Kingdom
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Transformers voltages are quoted for the current rating of the transformer, and a regulation % will also be quoted. So with no or light loading the nominal output rises by that percentage figure.
Your DAC will certainly have a voltage regulator... it won't work on the unregulated supplies, so within reason there won't be a problem.
9vac gives 12.7 volts DC after rectification, 10.28 volts gives over 16 volts DC.

Is that a problem... well it depends on the PSU in the DAC. If it uses 16 volt caps then yes, if the heatsinking on any regs is marginal, again yes there may be an issue.

Take a 5 volt reg passing 0.5 amps.
Input voltage 12.7 volts DC, power dissipated is 3.85 watts.
Input voltage 16.5 volts DC, power dissipated is 5.75 watts.

A big increase, and the reg would overheat if the heatsink were marginal.
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